Use today's date as default for Library Checks with an asOfDate param…

[joshwanf]

…eter

Closes #394

• Library Checks configured with blank asOfDate do not pass, since the Check is evaluating with an empty date.
• Today's date is injected in builder-api before calling library-api if the date is blank.
• Text is supplied in the UI that tells the user a default date will be supplied if left blank on the following screens:
  • Check Config screen when configuring parameters
  • Preview screen when displaying Results